1. The Field of the Invention
The present invention relates to servo algorithms for direct access storage drives. More specifically, the present invention relates to an improved servo algorithm for tangential misalignment precompensation in a direct access storage drive.
2. The Relevant Technology
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (DASD), such as a hard disk drive, incorporating rotating magnetic disks is commonly used for storing data in magnetic form on concentric, radially spaced tracks on the disk surfaces. Transducer heads driven in a path generally perpendicular to the drive axis are used to write data to the disks and read data from the disks.
DASD systems require a method to position each data head over the proper radial location for writing a track and for returning the data head very close to the same location when reading the track. With current DASD systems using a voice coil type of actuator, a control device with a feedback response is provided to locate and stably hold the head on a given track. Typically, track accessing (seeking) and track following (tracking) are enabled by magnetically written patterns on the disk surface. The patterns generally take the form of prerecorded tracking servo identification (SID) marks. The servo marks are read by the transducer head and transmitted to a control unit, which utilizes the servo marks to set and correct the position of the transducer head.
Several different basic servo schemes exist. A dedicated servo system reserves a dedicated surface of one of the disks in the DASD and records servo marks and other access information thereon. A sector servo system uses small portions of tracks between sectors of each track of each data surface to provide the servo mark and access information. A hybrid servo system uses both to obtain advantages of each type.
One problem all servo tracking systems must deal with is misalignment of the tracks after the servo marks have been placed on the disk surfaces. This misalignment results in a difference in an actual location of data written to a disk surface from the expected location as marked by the servo marks. Track misalignment generally arises from occurrences such as thermal differences between components, which causes nonuniform expansion or contraction of the components, slippage of the disks on the spindle which connects the disks, and shock inputs to the DASD unit. The shocks can result in xe2x80x9cspindle tilt,xe2x80x9d a relative tilting of the spindle on which the disks are aligned from an original axis.
These events result in two types of misalignment of the tracks relative to the servo marks. Misalignment that repeats is known as sinusoidal or xe2x80x9cACxe2x80x9d misalignment, because it is manifested as a repeating sinusoidal signal with a fundamental frequency equal to the speed of rotation of the disk. Misalignment that is constant along the entire track is often referred to as xe2x80x9cDCxe2x80x9d misalignment.
The misalignment is referenced in two directions relative to the transducer head. Misalignment occurring in a direction parallel to the direction of movement of the transducer head relative to the spindle is known as xe2x80x9cradial misalignmentxe2x80x9d or xe2x80x9ctrack misregistrationxe2x80x9d (TMR). Misalignment in a direction perpendicular to the direction of movement of the transducer head relative to the spindle is referred to herein as xe2x80x9ctangential misalignment,xe2x80x9d and results in errors in timing of the servo identification (SID) marks or signals.
Relatively slight misalignments are generally dealt with by the servo system using the position information from SID marks themselves. Nevertheless, more significant misalignments can occur that are outside the abilities of the head positioning servo system to compensate. Additionally, in order to quickly access the data tracks, the control unit may access information stored in memory regarding the location of the data tracks and generally, the location of the SID marks.
The prior art deals with the track misalignment in different manners in the radial and tangential directions. Radial misalignment, or repeatable radial offset (RRO), is currently precompensated for in certain instances with RRO feedforward and servo attenuation. Generally, this involves measuring the offset in the radial direction of the actual location of the SID marks from the expected location and storing the repeatable component of the offset in an RRO table. This offset is then fed by the control unit to the motor as a position error signal (PES), which supplements the servo control signals and corrects the positioning of the transducer head over the data tracks.
In the current art, there is no precompensation scheme for SID timing errors which occur from tangential misalignment. Instead, the art uses a technique of widening the opening of a SID timing window until the SID marks can be read. Under this arrangement, when disk shift has occurred, the SID marks do not appear when expected. The SID marks are written, as mentioned, between the sectors, (e.g., 96 times per track) with the data stored between the SID marks. When a SID mark does not appear where expected, the control unit opens the SID timing window in which the transducer head looks for the signal wider and wider until the SID mark appears. The default width for the SID timing window is selected to accommodate the expected range of tangential shifts of the SIDs for the entire DASD production run.
This arrangement has been found by the inventors to be increasingly problematic as recording densities increase. For instance, when the SID timing window is opened too wide, data in the data portion of the track is sometimes read and mistaken, due to a particular data arrangement, for the SID marks. This causes an error condition in the DASD, and sends the DASD into an error recovery mode, which significantly slows down the operation of the DASD. Widening the SID timing window for an entire production run of DASDs is also problematic, because so doing decreases the performance of the DASDs. Thus, a need exists in the art for overcoming problems that exist as a result of SID timing errors in higher density DASDs.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available servo systems in direct access storage devices. Thus, it is an overall objective of the present invention to provide a DASD servo system which precompensates for tangential misalignment and thereby avoids SID timing errors.
To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, a system and method for tangential misalignment precompensation is provided.
The method of precompensating for tangential misalignment generally comprises calculating tangential misalignment of the head with respect to the information located on the storage surface, and adjusting, in accordance with the calculation of tangential misalignment, a timing window during which the head reads the information located on the storage surface.
Both periodic (AC) and/or constant (DC) components of tangential disk shift can be calculated and compensated for, though AC disk shift is more common. When calculating AC tangential disk shift, the shift for a single track can be calculated and converted for other tracks.
In one embodiment, generating the calculation of tangential misalignment comprises converting a radial misregistration measurement into the tangential misalignment calculation. The calculation of tangential misalignment may be generated using an equation comprising:                               TMS          ⁡                      (                          r              ,              TMR              ,              t                        )                          =                              (                                          TMR                ⁡                                  (                                      t                    -                                          T                      /                      4                                                        )                                                            2                ⁢                                  xe2x80x83                                ⁢                π                ⁢                                  xe2x80x83                                ⁢                r                                      )                    ⁢          T                                    (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
Where TMR is the measured radial shift, t is the time at which the measurement is taken, T is the period of rotation, and r is the nominal radius of the track.
In a further embodiment, generating a calculation of tangential misalignment comprises directly measuring the tangential misalignment. Measuring the tangential misalignment may comprise measuring a difference between the time of a predicted occurrence of sensing the information located on the storage surface and the time of an actual occurrence of sensing the information located on the storage surface.
Adjusting the timing window in one embodiment comprises widening the timing window to accommodate tangential misalignment of the direct access storage device. In a further embodiment, adjusting the timing window comprises shifting the timing window in time to accommodate tangential misalignment of the direct access storage device. Preferably, the data stored on the storage surface comprises servo identification (SID) signals placed on the storage surface during manufacture of the direct access storage drive for locating the head in a desired location during read and write operations.
The tangential misregistration precompensation may be conducted on the fly for each track, but more preferably is conducted periodically for one track on each storage surface. Once the adjustments for one track are calculated, the proper adjustments for each other track on the surface can be computed therefrom using geometry and the respective radiuses of each track.
The method of precompensation for radial misalignment may be conducted by a servo system of a direct access storage drive (DASD). In one embodiment, the servo system comprises a magnetic recording disk provided with magnetically recorded data stored thereon and servo identification (SID) marks stored thereon identifying the locations in which the magnetically recorded data are stored, and a computer algorithm that uses the position information obtained from the SIDs to compensate for the apparent motion of the data tracks.
The DASD in which the servo system of the present invention is incorporated may also comprise a transducer head for reading and writing magnetically recorded stored on the magnetic recording disk and an actuator for moving the transducer head across the magnetic recording disk to access different regions of the magnetically recorded data on the magnetic recording disk. Additionally, the DASD may comprise a detector electrically coupled to the transducer head for detecting the magnetically recorded data.
A servo control unit is also preferably utilized within the DASD and in one embodiment comprises a tangential misalignment calculation module configured to generate a calculation of tangential misalignment of the head with respect to the information located on the storage surface. The servo control unit also preferably comprises a timing window adjustment module configured to receive the calculation of tangential misalignment and adjust, in accordance with the calculation of tangential misalignment, a timing window during which the head reads the SID information located on the disk surface.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.